The present invention relates to a method and apparatus for preventing a current collapse in a phase not participating in the commutation, of a three-phase, block current supplied synchronous machine while the phase currents of the other two phases commutate.
Synchronous machines with trapezoidal electromotive force (EMF) generate a constant torque if supply with block currents. Therefore, such machines are used in three-phase feed drives. The block currents are generated by means of a converter, where a rotating field is generated in the machine by means of a control unit and a control voltage. In addition, the torque is proportional to the machine current. It was found that torque collapses are caused by current collapses in a phase not participating in the commutation during the commutation of the two other phase currents of the three-phase block-current supplied synchronous machine.